Immunological memory to the lipololysaccharide of Escherichia coli 0113 (LPS 0113) was generated in strains of inbred mice given a single sub-immunogenic dose of either LPS 0113 or the native protoplasmic polysaccharide of E. coli 0113 (NPP 0113). Such memory, which involved, the synthesis of only antibody of the IgM class, developed in a cyclic manner that was characteristic for the strain of mice used. It involved cell proliferation as well as differentiation and persisted for at least 25 days after priming with a single injection of a sub-immunogenic dose of LPS 0113.